Just received this mail outlining the plan for extensions on AppSource:

Cumulative Update 1.3 for Microsoft Project “Madeira” is coming soon.

To: <madeira test account>

We hope you are enjoying your Project “Madeira” experience. As you may have read on our blog, an update is on its way to you. This includes new capabilities in the core application, and it also introduces marketplace support for extensions that was announced earlier today: https://appsource.microsoft.com.We are very excited about this new marketplace, and we hope that you will be as well. But unfortunately you will not be able to get extensions from AppSource just yet. We will let you know when your Project “Madeira” has been updated so you can use AppSource, it will only be a few days.

If you want to see what to look forward to in this update other than support for AppSource, take a look at our What’s New page.